The present invention relates to a design method of an optical element, such as an objective lens, used for an optical disc drive capable of recording data to and/or reproducing data from a plurality of types of optical discs based on different standards.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher than that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
Recently, new technical standards for optical discs, such as an HD DVD (High Definition DVD) and a BD (Blu-ray Disc), having further higher recording densities have been proposed. Such a new standard optical disc has a thickness of a cover layer smaller than or equal to that of the DVD. Under these circumstances, optical disc drives (i.e., optical systems in the optical disc drives) are required to support at least two types of optical discs. Hereafter, such an optical disc drive (objective lens) supporting at least two types of optical discs is frequently referred to as a compatible optical disc drive (objective lens).
For supporting a plurality of type of optical discs, an optical system for an optical disc drive is required to correct a spherical aberration which varies depending on the thickness of a cover layer of an optical disc being used, while changing a NA (numerical aperture) defining a beam spot size to a size suitable for the optical disc being used. In general, the beam spot diameter decreases as the wavelength of a light beam decreases. Therefore, a laser beam having a wavelength of about 660 nm, which is lower than a wavelength of about 780 nm for the CD, is used for the DVD, for example. For the new technical standard optical disc having higher recording density than the DVD, a laser beam having a shorter wavelength than that of the DVD (i.e., a so-called blue laser, for example, having a wavelength of about 408 nm) is used.
Furthermore, the optical system for the optical disc drive is required to suitably converge the light beam on a recording surface of an optical disc, on which the light beam of which wavelength varies depending on the type of the optical disc being used impinges, without causing a spherical aberration. For this reason, a conventional optical system of the compatible optical disc drive is configured to have a diffracting structure on one of surfaces of optical elements (e.g., an objective lens) in the optical system so that the light beam is suitably converged on a recording surface of each optical disc.
It is preferable that the optical element provided with the diffracting structure has the function of correcting a spherical aberration caused when a design wavelength of a laser beam varies depending on the change of an environmental condition, such as a temperature change or individual differences among light source devices. The term design wavelength means a wavelength of a light beam suitable for recordation and/or reproduction for an optical disc.
The degrees of freedom for design of a diffracting structure are limited. Therefore, it was assumed that if a diffracting structure is formed to support the two types of optical discs, giving additionally a correcting function for a wavelength shift (i.e., a function of correcting change of a spherical aberration caused by a wavelength shift in each of two different light beams for the two types of optical discs) to the diffracting structure is impossible.
Meanwhile, there is a demand for an optical disc drive capable of supporting the new technical standard optical disc as well as the existing optical discs (i.e., an optical disc drive capable of supporting three types of optical discs). However, the degrees of freedom of design of a diffracting structure are limited. Therefore, optical performance of a single diffracting structure is to converge, at the most, two types of light beams having different wavelengths on recording surfaces of the two types of optical discs, respectively. That is, if an optical system is configured to have a diffracting structure on one of surfaces of optical elements in the optical system, the optical system can support only two types of optical discs.
For solving such a problem, an optical system is proposed in an article “Compatible Objective Lens for Blu-ray Disc and DVD using Diffractive Optical Element and Phase-step Element which Corrects both Chromatic and Spherical Aberration”, Yoshiaki Komma et al. of Matsushita Electric Industrial Co., Ltd, ISOM collected papers (We-F-20). The optical system proposed in the article is configured to have different diffracting structures on two lens surfaces, respectively. By this configuration, the optical system is able to suitably converge the laser beams for the DVD and BD on recording surfaces of the DVD and BD, respectively, while correcting a spherical aberration caused by a wavelength shift in each of the laser beams for the DVD and BD.
However, if the optical system is configured to have the diffracting structures on two lens surfaces as proposed in the above mentioned article, the possibility that the optical performance deteriorates due to a manufacturing error, such as decentering, may increase. Further, if the optical system is formed by a single objective lens for the reduction in the number of components and manufacturing cost, the diffracting structures are formed on the both surfaces of the objective lens. In this case, one of the surfaces of the objective lens (i.e., one of the diffracting structures) is exposed to the outside through a disc tray, by which the possibility that the diffracting structure exposed to the outside is damaged when a lens cleaner is used increases.
Therefore, from a practical point of view, the optical system proposed in the article can not be used in the optical disc drive although the optical system is able to provide a plurality of types of diffracting functions for an optical element in the optical system.
Japanese Patent Provisional Publication No. 2000-171704 discloses a method of designing a single diffraction surface having a plurality of optical functions. The design method disclosed in the publication 2000-171704 is a method for giving a plurality of optical functions to a single diffraction surface so that a light beam having a particular wavelength can be used most suitably. However, in this publication, a design method for designing an optical element which can provide an optical disc drive (an optical system) with the capability of supporting a plurality of types of optical discs is not disclosed.